Spectral analysis is an important research method in natural science. Spectroscopy technology can detect physical structures, chemical compositions, and other indicators of detected objects. Spectral analysis is based on point measurement, image measurement is based on changes of spatial characteristics, and each of the two methods has its own advantages and disadvantages. Therefore, it can be said that spectral imaging technology is an inevitable result caused by development of spectral analysis technology and image analysis technology, and is a product of a perfect combination of the two technologies. Spectral imaging technology has not only the spectral resolution function but also the spatial resolution function to distinguish the image position. By spectral imaging technology, not only qualitative and quantitative analysis of a detected object but also positioning analysis of the detected object can be performed.
Multi-spectral imaging technology integrates images and spectrums, and can not only obtain geometry information of an imaging target but also identify spectral characteristics difference of the imaged target. Spectral characteristics of targets, false targets, and backgrounds are different, and have different spectral brightness in different spectral channels. Multi-spectral imaging can enhance a contrast between a target and a background, and thus can effectively suppress background noise, distinguish true targets and false targets, and improve system target identification ability. Multi-spectral imaging technology has already been widely used in various fields, such as aeronautical and aerospace remote sensing, land resources survey, forest fire prevention, environmental monitoring, marine applications, agricultural yield estimation, and so on.
Multi-spectral imaging technology uses a group of optical filters to divide the visible spectrum into at least five spectral bands, and each of the spectral bands corresponds to one channel. A target object is imaged in these channels respectively, and monochrome images of all the channels are combined together to form a multi-spectral image. Therefore, this technology is also widely used in the fields of scene simulation, natural light spectrum restoration, and spectral color measurement, and so on.
Optical filters and imaging lenses used in multi-spectral imaging have different refractive indexes in different spectral bands, which results in differences between equivalent focal lengths for imaging in these spectral bands. Therefore, only if each of these spectral bands is focused independently, can clear multi-spectral images be formed.
Conventional focusing devices can be divided into two categories.
(1) Focusing devices use specific mechanical devices to drive stepper motors to adjust distances between different lenses or distances between lenses and camera photoelectric detectors, and thereby achieve the focusing function. This type of focusing devices is widely used in ordinary cameras.
(2) Focusing devices use specific mechanical devices to maintain distances between lenses and cameras and adjust distances between cameras and photographed objects, and thereby achieve the focusing function. This type of focusing devices is generally used in microscopes.
In multi-spectral imaging, focusing devices of the category (1) can be adopted. However, multi-spectral imaging has many spectral bands, and when each of the spectral bands is accurately focused, conventional focusing methods have low efficiency. Therefore, a fast focusing method and a corresponding focusing device for multi-spectral imaging need to be developed, so that multi-spectral focusing can be achieved efficiently and clear multi-spectral images can be obtained.